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Is it possible? If not, why not?
It's not, you get:
You are only allowed to vote 1 time in this poll http://mrozekma.com/so-poll-one-time.png
You can post a feature request on the main meta and it might be changed
the question was: "Why not?" which there seems to be hesitance to answer this. If people could retract votes, which technically is possible, they could have themselves represented better, but they may not know best themselves.